1. Field of the Invention
The present invention relates to a nonaqueous electrolyte battery, a battery pack and a vehicle.
2. Description of the Related Art
Nonaqueous electrolyte batteries, in which lithium ions are transferred between a negative electrode and a positive electrode to charge and discharge, have been enthusiastically researched and developed as high-energy density batteries.
Nonaqueous electrolyte batteries using a lithium-transition metal oxide as a positive electrode active material and a carbonaceous material as a negative electrode active material have been already commercialized. Generally, Co, Mn, Ni or the like is used as the transition metal component of the lithium-transition metal oxide.
A nonaqueous electrolyte battery using a lithium-titanium oxide as the negative electrode active material has been recently put to practical use. The lithium-titanium oxide has a lithium ion insertion potential of about 1.55V vs Li/Li+, which is higher than that of a carbonaceous material. The lithium-titanium oxide is small in the change of volume accompanying the charge-discharge operation of the battery, the lithium-titanium oxide is expected to impart excellent charge-discharge cycle performance. Also, lithium ions are inserted in and released from the lithium-titanium oxide by a reaction that no lithium metal is precipitated on the negative electrode and therefore, it enables charge under a large current. Specifically, a rapid charge can be performed.
Note that this lithium-titanium oxide has low conductivity and it is therefore preferable to blend a conductive agent to enable charge and discharge under a large current. In JP-A 2003-163029 (KOKAI), the conductive agent is added to a negative electrode using lithium titanate as an active material, and ethylene sulfite and propane sultone are added to an electrolysis solution, thereby improving the negative electrode.
If a battery using a carbonaceous material as a conductive agent is stored under a high-temperature circumstance, this causes significant generation of gas. This is because a stable film is not formed on the carbonaceous material, which is a conductive agent, at a working potential of a negative electrode using lithium titanate as an active material, and the electrolysis solution is always decomposed and consumed on the surface of the carbonaceous material which is in contact with the electrolysis solution.
In JP-A 2005-317508 (KOKAI), on the other hand, a chain sulfite such as diethyl sulfite or dimethyl sulfite is added to the electrolysis solution to form a film on a surface of the negative electrode, thereby preventing gas from being generated from the negative electrode. However, such a film hinders the transfer of lithium ions or electrons on the surface of lithium titanate, giving rise to the problem that the large-current performance (high-rate load characteristics) of a battery is deteriorated.
Note that JP-A 2005-72008 (KOKAI) discloses that a negative electrode active material consisting of vanadium oxide represented by LixMyVzO2+d has a pore volume having a diameter of 0.1 to 10 μm in a ratio of 10−3 cc/g to 0.8 cc/g per particle weight.